The needs for fluid storage vessels are numerous going from general industrial/commercial, to process plants, and residential uses. There are a multitude of various fluids that need to be contained with their accompanying temperatures and pressures, thus creating a wide range of fluid storage vessel applications. Further, fluid storage vessel applications also typically require that the vessel be horizontally or vertically mounted; being mounted above ground, on the ground surface, or below ground. When vessels become large, i.e. storing thousands of gallons of fluid, wherein the vessel is literally large enough to allow an individual to walk inside, the stresses that the vessel experiences are quite large in magnitude. These stresses result from several areas; first from differential force or pressure loading from the weight and/or the inherent pressure of the fluid disposed within the vessel, second from the weight of the medium that is external to the vessel (i.e. such as a vessel is buried within the earth below the ground surface), third from contact with the structural supports that hold the vessel in a desired position, and fourth from the various fluid connections causing attachment moments through the vessel wall.
However, the primary vessel stresses of concern are the differential wall forces that the vessel experiences, from the weight or pressure of the fluid disposed within the vessel interior or the weight or pressure of the external medium acting against the external walls of the vessel (i.e. for example in the case of a vessel buried beneath the ground surface). For a typical vessel, the basic shape is that of a cylinder which from the interior of the vessel experiences basically two types of stress; the first being the hoop stress and second being the axial or long stress. Hoop stress is the force against the curved sidewalls of the vessel which project in a flat plane of area roughly equal to a lengthwise cut through the vessel and grow with increases in the diameter. Long stress is perpendicular to the hoop stress being the force against the ends of the vessel that is parallel to the longitudinal axis of the cylinder. For a given cylinder shape the hoop stresses increase with the diameter of the cylinder, wherein the long stress is not a function of cylinder length along the longitudinal axis.
This cylinder stress relationship between the hoop and long stresses leads to some optimal configurations for cylinders depending upon the application, such that a cylinder containing a higher internal pressure is optimally small in diameter and longer in length, as the diameter increases high wall stress (i.e. larger diameter equals higher stress) wherein a longer length cylinder does not add to wall stress. Thus a cylinder that is short in length and a cylinder that is long in length experience the same wall stress from internal loads. The key to adding internal volumetric storage capacity is to keep the diameter minimal and to gain the internal volumetric capacity from increases in cylinder length, although the aforementioned long stresses must be considered that come with a longer small diameter cylinder design. As for forces external to the vessel cylinder, that magnitude of the forces are similar to internal cylinder pressure, (i.e. a larger diameter increases the external forces, while increases in cylinder length do not add to the external forces in the horizontal position). However, the wall stress effect on the cylinder from internal versus external force are different, as the external compression forces such as earth loading introduce bending moments in the vessel wall that can complicate the strength analysis, as opposed to the more pure tension stresses that internal fluid loads create on the wall of the vessel.
In so far as the materials of construction are concerned for vessels, various materials have been used in the past to construct vessels all having various advantages and disadvantages. In the past, the more common materials of construction have been steel and concrete, however fiberglass is gaining more and more popularity especially due to its anti-corrosion properties as against the internal fluid as well as any external medium. Steel tanks are typically prone to rusting, (unless they are constructed of stainless steel, which is typically not done due to high cost) especially when exposed to groundwater or above ground wet weather. Concrete does not rust of course, but may develop hair line fractures and is typically porous in nature leading to issues with absorbing internal fluids and deterioration over time. Fiberglass has good resistance to corrosion, but is relatively brittle, requiring careful handling, especially during shipping and installation. A sharp blow or inadvertent vessel point contact can easily cause considerable damage to a fiberglass vessel.
Both steel and concrete tanks are relatively heavy. This typically results in the tanks being constructed near or at the point of installation to reduce the energy cost of transportation and related installation difficulties. The weight of steel and concrete vessels effectively limits the maximum size of a vessel which can be transported by common carriers over the interstate highways or railroads. On-site or field construction greatly adds to the labor cost and time required for such steel or concrete vessels. Fiberglass has some attractiveness in this area as a much lighter material which can be used to mass produce vessels in a controlled factory environment. A fiberglass vessel can be relatively large, light weight, and easier to ship and install. However, considering the prior difficulties associated with dropping, bumping, or impacting the relatively brittle fiberglass vessel can be difficult to overcome, especially since the repair of a damaged fiberglass vessel on-site can be technically difficult and costly.
An alternative vessel construction material is a high density Polyethylene which offers many of the positive aspects of fiberglass, such as the light weight and anti-corrosive properties. Polyethylene vessels are typically formed into cylindrical type shapes using a rotary molding process which produces a one-piece, seamless tank. The advantages of polyethylene are its softer and more flexible nature as compared to fiberglass. Polyethylene vessels are far more impact resistant and will flex rather than crack when the polyethylene vessel is subjected to shipping and installation irregularities, bumping and so on, as previously described. However, the drawback of this softer polyethylene material is that it is structurally weaker, which is a major design consideration. Looking at the aforementioned discussion related to vessel stresses, the polyethylene lower flexural modulus issue must be dealt with carefully in the design process.
The shipment of factory made vessels is severely limited to what a typical a flatbed truck can carry. In many situations the internal volume or internal capacity required often exceeds the shipping size that a flatbed truck can effectively deliver. One solution is the use of segmented vessels, wherein a number of smaller modules can be assembled together to add the desired internal volumetric capacity. However, a vessel's segmented construction presents assembly, alignment, and fluid sealing issues that must be dealt with at the location where the tank is to be installed.
The present invention deals with an apparatus to selectively heat primarily water storage vessels that are utilized for water storage used for fire protection, drinking, and a multitude of other uses, wherein the vessel is typically an on-site built type constructed of steel with a concrete foundation with the vessel being ground surface mounted and shaped as a vertically oriented cylinder that is fairly large in volume being in the hundreds of thousands of gallons range.
As the availability of the stored water is paramount year round, in geographic areas where the environmental air temperature can drop below freezing, provisions must be made for keeping the stored water from freezing being either thermal or chemical, wherein for maximum applications for use, the thermal route is most often used as manifested by a water heating appliance that can be fuel based or electrically based. Fuel based heaters are usually more efficient but have higher initial cost and higher installation cost, whereas electrically based heaters are usually less efficient, however, having lower initial cost and lower installation cost. Thus resulting in the economies such that a heater that is occasionally used would be typically an electric heater and a heater that is fairly continuously used would be typically be a fuel based heater, such that for a seasonal use tank water heater (being an occasional use for winter months only) would normally be an electric based water heater.
In analyzing the above, there are numerous ways to accomplish water tank heating, depending upon the severity of the potential water freezing, the type of tank (size, construction, configuration, etc.), the use of the tank water, cost, installation, and maintenance issues. What follows are some examples of tank heaters in the prior art having different applications or uses and their accompanying differing heater mounts, installation, and maintenance issues.
In looking at the prior art in this area, in U.S. Pat. No. 4,883,943 to Davis disclosed is an electric heater for a fuel tank that is disposed within the tank drain (outlet) in an application for a diesel truck to prevent cold weather fuel waxing as this is the most common application wherein the heater is located within the tank outlet, with the diesel fuel warmed at the point wherein it is pumped into the diesel engine injectors. In looking at FIGS. 1, 3, and 4, of Davis the heater is positioned in the outlet of the tank, however, it is also partially disposed within the tank interior volume itself, also heater rod replacement would require tank fluid or fuel draining, which on a vehicle probably is not as big of deal, as it would be a more difficult proposition in a very large permanent ground surface mounted storage tank.
Continuing, in the tank heater prior art in U.S. Pat. No. 6,810,206 to Clark, Jr., disclosed is a drain plug mounted heater for an application or field of use in livestock water tanks that typically have an open top, wherein it is convenient to mount the heater in the drain opening, however, noting that the heater is an immersion type, i.e. such that it is merely using the drain port opening to physically mount the heater while the heater is immersed into the tank interior. The primary novelty in Clark is in the plug being able to pass therethrough the relatively small drain opening, however, the heater being much larger that the tank drain opening size, pointing to the advantage of an open top smaller tank, wherein the tank can be drained and the heater installed from inside the tank from the open tank top.
Next, in the tank heater prior art field in United States Patent Application Publication Number 2012/0175358 to Davidson, Jr., disclosed is an automotive engine oil pan drain plug heater, being the other common heater in the tank drain application, wherein the heater threads into the oil tank drain for keeping the oil viscosity lower in cold weather, wherein the heater inserts into the tank interior from the outside therethrough the drain opening, thus requiring removal of the heater to drain and change the oil from the oil tank.
What is needed is a relatively small-truck transportable, lightweight, segmented modular type enclosure that can be easily transported and installed in its permanent location while easily fitting on a typical truck with the assembled segments being light in weight and small enough in size to avoid high capacity crane and specialized rigging equipment being required for the tank heater installation and maintenance. Further, issues that need to be addressed are the additional problems of alignment, attachment, and sealing that accompany a segmented apparatus enclosure design suitable for fast field assembly, wherein the apparatus supports and contains the heater being in fluid communication with the tank fluid. Further, the apparatus can accommodate heater removal and re-installation of the heater for maintenance reasons without the need to disturb or drain the fluid in the tank. Another desirable benefit of the heater/apparatus assembly would be to enhance the thermal effect of the heater disposed within the apparatus via the fluid communication with the tank fluid to diffuse the heater output into the tank fluid using thermal conduction and convection primarily to increase the efficiency of the heater. Thus in summary, the heater is disposed completely outside of the interior tank volume and that the heater can be serviced or replaced without draining the tank fluid, while at the same time providing adequate heating to facilitate water flow from the tank in freezing exterior temperatures.